Increased use of consumer electronics such as cellular telephones, laptop computers and other portable devices, and the development of new technologies like electric vehicles (EV) has increased the demand for compact, durable, high capacity batteries. This demand is currently being filled by a variety of battery technologies including traditional lithium ion batteries. However, the metal packaging of traditional batteries makes them heavy, thick, prone to leakage and difficult to manufacture. A new generation of solid-state batteries is emerging that allow the fabrication of consumer batteries in a wide variety of shapes and sizes that are thinner, safer and more environmentally friendly. However, state of the art, solid-state batteries have several shortcomings including relatively low values of ion conductivity.
Lithium polymer electrolytes have received considerable interest for use in solid-state batteries. These electrolyte systems are complex materials composed of amorphous and crystalline phases. It has been known since 1983 that the ion motion in polymer electrolyte occurs predominantly in the amorphous phase. Accordingly, the conventional approach to improving ionic conductivity has been to investigate conditions that either decrease the degree of crystallinity or increase the segmental motion of the polymer matrix. However, despite significant improvement, modern lithium-ion batteries employing polymer electrolytes are limited by lithium ion conductivities of order 10−6 S cm−1 at ambient temperatures. This level of conductivity is not sufficient for many consumer battery applications.
The 10−6S cm−1 conductivity ceiling was overcome by true solid-state batteries developed by Duracell in the 1970s which used pressed aluminum oxide (Al2O3) powder and Li salt (LiI) as the electrolyte material. See, U.S. Pat. No. 4,397,924 issued to Rea on Aug. 9, 1983 (Rea '924). The solid alumina electrolyte provided two orders of magnitude greater conductivity than polymer electrolytes due to the hopping mechanism by which lithium ions can travel across the surfaces of alumina particles by hopping from oxide oxygen to oxide oxygen on the amorphous surface. (Kluger K, Lohrengel M, Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 95 (11): 1458-1461 NOV (1991)). However, this ion conduction only occurs when sufficient contact between the two alumina particles is both created and maintained. The Rea '924 patent overcame the first part of the contact problem by severely compressing the components at compressive strengths in the order of 100,000 psi. The result is a very dense solid-state electrolyte. However, overtime the ionic conductivity of the electrolyte decreases as the contact between particles degrades. This is especially true when the electrolyte is subjected to shock or other trauma. Because Rea relies on physical compression to create contact between alumina particles, very small change in the contact between the alumina particles has a profoundly negative effect on the ion conduction of the material. In fact, this technology was virtually abandoned because of this limitation.
Recently aluminum oxide (Al2O3) membranes have been considered for use as battery materials by other researchers, however, the mechanism for lithium-ion conductivity of the membrane itself has neither been considered nor explored, nor has the modification and adjustment of the membrane. For example, U.S. Pat. No. 6,586,133 issued to Teeters et al., on Jul. 1, 2003 (Teeters '133) teaches a nano-battery or micro-battery produced by a process comprising: providing a membrane with a plurality of pores having diameters of 1 nm to 10 μm, filing said membrane with an electrolyte; and capping each filled pore with an electrode from about 1 nm to about 10 μm in diameter in communication with said electrolyte to form individual nano-batteries or micro-batteries. While Teeters '133 suggests the use of aluminum oxide membranes, it teaches the membranes solely as a “jacket” for nano or micro cells. The Teeters patent is directed solely to the creation of nano and micro batteries and never teaches or even suggests using the membrane to enhance the ion conductivity of the electrolyte in a synergistic manner. For example, the preferred pore diameter range of Teeters' system is much too large for meaningful ion conductivity enhancement by the metal oxide membrane. Furthermore, Teeters teaches the use of AAO membranes with low pore densities and porosities which are inadequate for producing effective active membranes. Teeters also teaches that the anode and cathode material of the preferred embodiment are contained inside the pore of the AAO membrane. Teeters invention, can be fabricated equally well by employing a variety of materials having pores. The principle of Teeters is the miniaturization of a battery cell using AAO as a nano-container, not as a material for enhancing the performance of the battery itself.
Mozalev, et al. teach a porous alumina membrane as the separator for macrobatteries. See, A. Mozalev, S. Magaino, H. Imai, Electrochimica Acta, 46, 2825 (2001). Their work has suggested that alumina membranes mechanically suppress Li dendrite formation, thereby improving cycling efficiencies. However, they have not suggested or discussed the lithium-coordinating role aluminum oxide membrane walls can play, nor have they taught an optimization of this parameter. The object of the Mozalev invention is to mitigate formation of dendrites by use of a hard material for a battery separator. Any hard material will serve the object of Mozalev's invention.
A major breakthrough in the room-temperature conductivity of lithium polymer electrolytes would significantly impact the rechargeable consumer battery market, as well as the emerging electric vehicle (EV) arena. Despite more than 20 years of active industrial and academic investigation, the current level of conductivity for lithium polymer electrolytes is not sufficient for many battery applications and suggests that a radical new approach based on a better understanding of ion transport is required.